1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a lamp and a back light unit thereof adapted for improved brightness and efficiency. The present invention also relates to a liquid crystal display adapted for increased brightness and decreased power consumption using the back light unit.
2. Discussion of the Related Art
Liquid crystal displays have wide applicability because of their lightness, thinness, and low driving power consumption, by way of example. According to this upward trend, the liquid crystal display may be used in office automation equipment and audio/video equipment by way of non-limiting example. The liquid crystal display controls the amount of transmitted light in accordance with a signal applied to a plurality of control switches which are arranged in a matrix shape. Thus, a desired picture is displayed on a screen.
The liquid crystal display device is not a self-luminous display device, thus it requires a separate light source such as a back light.
Back lights may be classified into a direct type and an edge type depending on the location of a light source. The edge type back light has a light source installed at the edge of one side of a liquid crystal display, and irradiates an incident light from the light source to a liquid crystal display panel through a light guide panel and a plurality of optical sheets. The direct type back light has a plurality of light sources disposed directly under the liquid crystal display, and irradiates the incident light from the light sources to the liquid crystal display panel through a diffusion plate and a plurality of optical sheets. Recently, the direct type backlight of which brightness, light uniformity and color purity are higher than those of the edge type backlight, is more often used in LCD TVs.
The light source used as the back light may be a Cold Cathode Fluorescent Lamp (hereinafter, referred to as “CCFL”) and an External Electrode Fluorescent Lamp (hereinafter, referred to as “EEFL”), for example.
Referring to FIG. 1, a related art EEFL is comprised of a glass tube 10, a fluorescent material 12 coated at an inside wall of the glass tube 10, inactive gasses 14 (or discharge gas) injected within the glass tube 10 and an external electrode 16 installed at external sides of both edges of the glass tube 10.
The glass tube 10 may be elliptical with an internal diameter approximately 1.6 mm, an external diameter approximately 2.0 mm, and a length of the glass tube 10 approximately 50˜400 mm.
The inactive gasses 14 may be a combination of Ne and Ar having a constant ratio, and include an amount of Hg.
If an alternating voltage from an inverter is applied to a high pressure electrode and a low pressure electrode, then an electron emitted from the low pressure electrode of the EEFL collides with the inactive gasses within the glass tube to thereby exponentially increase the quantity of electrons. The inactive gas is excited by the electrons and emits ultraviolet rays. The ultraviolet rays collide with the fluorescent material coated at the internal wall of the glass tube to emit visible rays.
In the EEFL, the length of the glass tube 10 has been increasing because of the increased size of the liquid crystal display. If the length of the glass tube 10 increases then thickness of the glass tube and diameter of the glass tube 10 are increased. In such a discharge tube, if the thickness of the glass tube 10 is increased, then brightness and efficiency of the lamp decrease, and if the length of the glass tube is increased, then distance between electrodes is increased. Thus, driving voltage for generating discharge is increased. Furthermore, if the diameter of the glass tube 10 is increased, then brightness of the lamp is decreased. The liquid crystal display using a back light unit having a low brightness and poor efficiency of the lamp as a light source results in low brightness and high power consumption.